Mice fed a fatty diet begin eating more during the day, when mice — being nocturnal — are supposed to be asleep. They also exhibit changes in the molecular components of the circadian clock and in important aspects of metabolic chemistry.

“We found that, as an animal on a high-fat diet gains weight, it eats at the inappropriate time for its sleep/wake cycle. All of the excess calories are consumed when the animal should be resting,” said Joe Bass of Northwestern University. “For a human, that would be like raiding the refrigerator in the middle of the night and binging on junk food. You can begin to see changes in the animals’ daily habits very rapidly—within a matter of days.”

In the study, the mice received 45 percent of their calories from fat. For humans, it’s recommended that no more than 30 percent of calories come from fat. After two weeks on the high-fat diet, the animals showed significant behavioural changes. Their daily sleep/wake cycle grew longer, suggesting that the central mechanism in the brain that controls the timing of activity and rest had been affected. That observation was made in animals kept in constant darkness, so that their behaviour reflected only the workings of their internal clocks.

The researchers then examined the animals’ activity patterns when kept in 12 hours of light and 12 hours of darkness. As expected, mice fed a regular diet maintained a robust daily rhythm of food intake and activity throughout the entire experiment, with most of their activity and feeding, about 80 percent, occurring during the dark period. As early as the first week on the high-fat diet, the mice began consuming more food when the lights were on.

In addition, the researchers found, mice on the high-fat diet displayed altered activity of key genes that control the roughly 24-hour circadian rhythm. The high-fat diet suppressed the activity of the core clock genes, Bass said.